1. Field of the Invention
The present invention relates generally to defibrillation methods, and more particularly, to implantable defibrillation systems for optimizing the efficiency thereof by steering energy and current.
2. Description of the Prior Art
Defibrillation, or causing the cessation of chaotic and uncoordinated contraction of the ventricular myocardium by application of an electrical voltage and current, in its most primitive form goes back to the last century. (J. L. Prevost and F. Batelli, "Sur Quelques Effets des Descharges Electriques su le Couer des Mammifers", Comptes Rendus Hebdomadaries des Seances de L'Acadmie des Sciences, Vol. 129, p. 1267, 1899.) The sophistication and effectiveness of defibrillation techniques has grown rapidly in subsequent decades. One of the most recent developments has been the practical advent of implantable defibrillation systems. (R. A. Winkle, et al., "Long-term Outcome with the Implantable Cardioverter-Defibrillator", J. Am. Coll. Cardiol., Vol. 13, p. 1353, 1989; M. H. Lehman and S. Saksena, "Implantable Cardioverter-Defibrillators in Cardiovascular Practice: Report of the Policy Conference of the North American Society of Pacing and Electrophysiology", PACE, Vol. 14, p. 969, June, 1991.) With the acceptance of this technology, the new challenge is to reduce system size while preserving its effectiveness, in order to improve the patient's quality of life and to extend the range of application of such systems. (R. A. Winkle, "State of the Art of the AICD", PACE, Vol. 14, p. 961, May, 1991, Part II; N. G. Tullo, S. Saksena and R. B. Krol, "Technological Improvements in Future Implantable Defibrillators", CARDIO, Vol. 7, p. 197, May, 1990.)
Until an ability to anticipate fibrillation has been achieved, it will be necessary to achieve defibrillation by passing a large current through the heart. The current must be large enough to depolarize a large fraction of the myocardium, thus extinguishing depolarization wavefronts. (D. P. Zipes, et al., "Termination of Ventricular Fibrillation in Dogs by Depolarizing a Critical Amount of Myocardium", Am. J. Cardiol., Vol. 36, p. 37, July, 1975.)
The high values of current that are employed generally in defibrillation procedures and the compactness that is essential in implantable systems are conflicting requirements. For this reason, a huge premium is placed on optimal delivery of the current and energy necessary for successful defibrillation. But, with the kinds of defibrillation electrodes that are most conveniently employed, the energy distribution or "steering" that occurs naturally departs significantly from the optimum. The present invention provides steering methods that improve the pattern of energy delivery when the simplest and least costly electrode systems are used.
For reasons of simplicity and compactness, capacitor-discharge systems are almost universally used in defibrillation. Achieving the requisite electric field needed to depolarize most of the myocardial cells requires current density above a certain threshold value, and via Ohm's law, this means the process is favored by achieving sufficiently low electrical resistance in the discharge path. For this reason, the use of electrodes of relatively large surface area has for a long time been the norm. (A. C. Guyton and J. Satterfield, "Factors Concerned in Defibrillation of the Heart, Particularly through the Unopened Chest", Am. J. of Physiology, Vol. 167, p. 81, 1951.) The discharge of a capacitor C through a fixed resistance R results in a voltage-versus-time curve (and hence, current versus time as well) that is a declining exponential, with a characteristic time given by the product RC, as illustrated in FIG. 1A. Although the heart does not constitute a "fixed" resistor (that is, does not have a linear current-voltage characteristic) the assumption of its linearity yields a useful qualitative description of defibrillation electrical events, and will be used in the explanations that follow.
It has also been recognized for some time, however, that the low-voltage (and low-current) "tail" of the capacitor-discharge pulse is detrimental. (J. C. Schuder, G. A. Rahmoeller and H. Stoeckle, "Transthoracic Ventricular Defibrillation with Triangular and Trapezoidal Waveforms", Circ. Res., Vol. 19, p. 689, October, 1966; W. A. Tacker, et al., "Optimum Current Duration for Capacitor-Discharge Defibrillation of Canine Ventricles", J. Applied Physiology, Vol. 27, p. 480, October, 1969.) The exact reason for this detrimental effect is not known, although plausible speculations exist.
For this reason, most efforts have been directed at time-truncated capacitor discharges, as illustrated in FIG. 1B. (J. C. Schuder, et al., "Transthoracic Ventricular Defibrillation in the Dog with Truncated and Untruncated Exponential Stimuli", IEEE Trans Biom. Eng., Vol. BME-18, p. 410, November, 1971.) That is, the capacitor discharge is simply interrupted by opening a switch at some middle point, typically, approximately at the time that the characteristic "RC time" has been reached. The advent of compact solid-state switches has made such pulse tailoring a straightforward matter. The truncated capacitance-discharge pulse shown in FIG. 1B is known as the monophasic waveform of the prior art. An important variation on the pulse shape is the biphasic waveform of FIG. 1C, wherein the pulse is extended, but with reversed polarity, again through a switching technique.
Now turn to the electrodes used to deliver current to the cardiac muscle. Among the most efficient are epicardial-patch electrodes 46 and 47, which are relatively large-area flexible conductive elements that are stitched directly to the epicardium, or exterior of the heart 49, as illustrated in FIG. 2. The obvious shortcoming of this method is, of course, that it requires major surgery and invokes the hazards of opening the chest cavity for attachment of the patches.
An appreciably simpler option positions an endocardial coil electrode 62 at the right ventricular apex (RVA) 64, or the lowest point inside the right ventricle 66. This usually takes the form of a helix or coil, fabricated from metal, ribbon or wire, wound on the exterior of a cardiac catheter 60 that is introduced intravenously. This option is illustrated in the lower-left hand portion of FIG. 3. Since the catheter can be used for other purposes, such as pacing, as well, the simple addition of an electrode to it is an attractive feature of this option. Even more important, the surgery needed for the use of epicardial patches is avoided. And since it is ventricular tissue that requires electrical treatment for successful defibrillation, such an RVA electrode 62 is quite strategically positioned for the purpose. A second electrode is also needed, of course, to accompany the RVA electrode. An option that is often used is the subcutaneous patch 54 (hereinafter abbreviated as "SUB"). It is a relatively large-area element, illustrated schematically in the lower-right portion of FIG. 3 (where distances are not represented to scale), that is positioned under the skin of the left chest. Thus, current passed from SUB 54 to RVA 62 intersects virtually all of the ventricular tissue, as desired. The SUB 54 option has the shortcoming that the patch, as a shallow implant, is vulnerable to damage from bumps and other mishaps, and even from normal bodily motion, but it has the major advantage that open-chest surgery is again avoided.
A promising alternative (for augmentation) to the SUB electrode is using the metal housing or CAN 52 (as hereinafter designated) of the defibrillation pulse-generator module 57 as an electrode, an option that is the subject of a co-pending application, now issued as U.S. Pat No. 5,199,429. This option is especially attractive because CAN 52 is mechanically stronger than SUB 54, and hence is not liable to breakage, and also because CAN 52 must be implanted in any case. The CAN option is illustrated schematically in the upper-right portion of FIG. 3.
This use of the pulse generator itself brings the proviso, however, that CAN 52 must be small enough for pectoral implantation without serious comfort or cosmetic compromises. The larger implantable pulse generators of the prior art have been placed in the comparatively spacious abdominal cavity. But abdominal placement does not permit use of the CAN electrode concept; the current path is too long for efficiency, and also the discharge would result in extreme discomfort. Further, the current vector intersects the heart disadvantageously. Thus, a further premium and benefit is derived from a size reduction of the pulse generator that permits pectoral implantation, and the efficiency improvement inherent in the present invention is aimed at precisely such a size reduction.
A fourth electrode option that is comparatively easy to realize is simply a second endocardial coil added to a catheter that already carries the RVA electrode. (M. Mirowski, U.S. Pat. No. 3,942,536, 1976). It is a straightforward matter to position this second coil electrode 68 in the superior vena cava 70 (SVC hereinafter), the chamber immediately above the right atrium 72, as illustrated in the upper-left portion of FIG. 3. The right atrium itself is sometimes chosen as the location, and an appreciably more difficult-to-reach location, the coronary sinus, is also sometimes chosen. But since these three sites are proximate, we shall simplify the following descriptions by using the SVC designation only to stand for any electrode in this region of the heart.